1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus and a gear transmission device including an internal gear, and more particularly to a process unit including the gear transmission device and an image forming apparatus including the process unit.
2. Description of the Related Art
A gear transmission device for driving a driven rotator such as a development roller or a photosensitive drum in an image forming apparatus has a gear train that is operated in combination of low rotation/high torque drive and high rotation/low torque drive. In the case of the low rotation/high torque drive, the number of teeth (i.e., a pitch circle diameter) of a drive external gear attached to a rotary shaft of a drive motor is set to be small in order to reduce a speed; whereas, the pitch circle diameter of a first-stage driven gear meshing with the drive external gear is set to be relatively large.
However, if the number of gear teeth of the external gear of the drive motor is reduced, an adequate contact ratio between the first-stage driven gear and the external gear cannot be secured, thereby causing irregular rotation, noise, and vibration. Although such noise or vibration can be suppressed to some extent by injecting grease into a mesh portion between gears, it is not enough. Furthermore, injecting grease for multiple times causes a higher risk of foreign substances getting into the mesh portion between the gear teeth, which results in noise, degradation of rotation accuracy, and damage to the gear.
A gear transmission device having a higher contact ratio and reduced noise and vibration is disclosed in, for example, JP-H11-311302-A. In this gear transmission device, a first-stage driven gear meshing with an external gear attached to a rotary shaft of a drive motor is constituted by an internal gear.
Meanwhile, JP-H09-114160-A proposes rotating two or more driven rotators at a predetermined rotational speed ratio in an image forming apparatus. In this image forming apparatus, the rotation of a drive motor disposed on a main body side is transmitted to a development roller and a photosensitive drum in a process unit via a plurality of gears disposed on the process unit side.
At the mesh portion between the gears, fluctuations in rotational speed occur due to flexure of teeth or the like. When the speed fluctuations are large, a toner image to be formed on a photosensitive drum may be influenced, thereby possibly degrading the quality of the image. In view of this, in the image forming apparatus disclosed in JP-H09-114160-A, a plurality of elastic ribs is disposed inside the gears on the process unit side in attempting to reduce the speed fluctuations as much as possible. In order to prevent any mutual influence of speed fluctuations between driven rotators, there has been known a gear transmission device in which two or more drive motors are used so that gear trains in the driven rotators are independent of each other.
Such a gear transmission device is effective in suppressing the speed fluctuations since the contact ratio between the internal gears disclosed, for example, in JP-H11-311302-A, is high. However, when the internal gear train of JP-H11-311302-A is employed in the process unit disclosed, for example, in JP-H09-114160-A, the distance between the shafts of the gears becomes large, and therefore, a large space is required, thereby complicating efforts to reduce the size of the gear transmission device or the process unit.
Alternatively, two drive motors may be disposed on the main body side of the image forming apparatus, and then, a development roller and a photosensitive drum may be rotated via separate gear transmission devices. In this configuration, when the gear transmission device is of an independent type, a load exerted on each of the gears becomes small, thereby suppressing speed fluctuations. In addition, the speed fluctuations are not influenced between driven rotators. However, when the drive motor and the gear transmission device are independently disposed in the driven rotators, it is difficult to reduce space, cost, noise, or vibration.